Vinyl chloride (xe2x80x9cVCxe2x80x9d below) is prepared industrially by thermal cracking of 1,2-dichloroethane (xe2x80x9cEDCxe2x80x9d below) at temperatures of 450 to 600xc2x0 C. at elevated pressure. In this process, about 40 to 65% of the feed EDC is cracked into VC and hydrogen chloride. In this cracking, by-products having higher and lower boiling points than the EDC are formed. Multistage separation by distillation must therefore be carried out.
The gaseous products of the thermal cracking (cracking gas) are first passed through a filter to remove relatively coarse coke particles, condensed and passed into the first distillation column (HCl column), in which the hydrogen chloride is distilled off as overhead product. In a further column (VC column) the VC is distilled off as overhead product. The bottom phase of this column comprises the unreacted EDC and lower- and higher-boiling by-products and relatively fine coke particles.
These coke particles interfere in particular with the necessary removal of the benzene, so that to date complex fractionation of the bottom product and purification of the partial stream which were fed to the benzene removal stage have been necessary. Preferably, this fractionation and partial purification were performed by flash evaporation.
DE-A-40 33 047 discloses a process for removing benzene from the EDC recovered during VC preparation, after the separation of VC and hydrogen chloride, which is characterized in that the EDC from the column bottom of the VC column is divided into a product stream purified from high-boiling substances and an unpurified product stream, the purified product stream is treated with chlorine in the presence of metallic iron at a temperature of 30 to 85xc2x0 C. and this product stream is recombined with the unpurified product stream and the EDC is conjointly separated off from the high-boilers in a further purification step. An improvement of this process is that the catalyst used is xcex3-aluminium oxide (DE A-41 2 9 391).
DE-A-41 39 632 further discloses a process for removing benzene from the EDC recovered in the VC preparation downstream of the separation of the VC and hydrogen chloride, which is characterized in that the contaminated EDC from the column bottom of the VC column is separated into a product stream I freed from high-boiling substances (b.p. greater than 83.7xc2x0 C. 760 mmHg) and a product stream II containing the high-boiling substances (b.p. greater than 83.7xc2x0 C. 760 mmHg), the product stream I is treated with chlorine in the presence of metallic iron at a temperature of 30 to 85xc2x0 C. and the treated product stream I is combined with the product stream II and the combined product streams are conjointly treated with chlorine at a temperature of 30 to 85xc2x0 C. and purified EDC is separated off from this product stream in a distillation zone.
DE-A-41 32 761 discloses a process for working up the cracking gas which is produced in the thermal cracking of EDC to give VC at a temperature of 480 to 540xc2x0 C. and a pressure of 15 to 25 bar and is cooled to a temperature of 180 to 280xc2x0 C. if appropriate in a heat recovery stage and at this temperature is passed into a quench zone in which it is cooled and scrubbed by condensed cracking gas, which is characterized in that a) 80 to 99% by weight of the cooled cracking gases are taken off in the gaseous state as overhead product and 1 to 20% by weight of the cooled cracking gases are taken off as liquid bottom product of the quench zone, b) the coke present in the bottom product of the quench is comminuted, c) the dispersion produced in this case is separated in a stripping zone into a distillate and a bottom phase and the bottom phase from the process is ejected and d) the distillate of the stripping zone is combined with the gaseous overhead product of the quench zone after its condensation. Preferably, 3 to 6% by weight are taken off as bottom product from the quench zone.
It has now been found that the following mode of operation of the quench is suitable for considerably simplifying benzene chlorination: the fractionation of the product streams and the purification of one of these product streams can then be omitted, if the gaseous products from the thermal cracking of EDC are cooled to a relatively narrow, low temperature range of about 150 to 180xc2x0 C., preferably 160 to 170xc2x0 C., so that, in this condensation stage, only a small proportion of EDC, together with high-boiling fractions and coke formed, condenses. The majority of the unreacted EDC is thus passed together with the products VC and hydrogen chloride to the work-up by distillation, firstly the hydrogen chloride, and in a further stage, the VC, being separated off from the EDC by distillation, in a manner known per se. The residual EDC can then, without purification, be passed to the plant for removing benzene which is expediently performed using xcex3-aluminium oxide in accordance with the process of DE-A-41 29 391.
It is generally sufficient to subject only a portion of the EDC to the benzene chlorination in order to obtain a sufficiently low content of benzene and its secondary products in the EDC and VC. However, for the fractionation into partial streams, according to the invention a simple apparatus such as a control valve is sufficient.
According to the invention, less than 7%, preferably less than 5%, of the EDC can be condensed, so that only a relatively small proportion of coke-containing condensate must be worked up separately. Furthermore, the flash evaporation required in the known benzene chlorination process can be omitted, so that the process according to the invention is considerably less complex.